Communication signals are transmitted at a desired frequency obtained by multiplying the original information carrying base band signal by a carrier frequency. In a Radio Frequency (RF) transmitter, for example, the desired RF frequency is typically obtained from the information carrying digital base band signal by converting the digital signal to an analog signal, and then mixing the analog signal with an RF carrier frequency signal using one or more mixers.
A number of direct synthesis techniques have been proposed or suggested for directly synthesizing RF signals from the information carrying digital base band signal. For example, International Patent Application Serial No. PCT/US09/38929, filed Mar. 31, 2009, entitled “Methods and Apparatus for Direct Synthesis of RF Signals Using Delta-Sigma Modulator,” discloses techniques for direct synthesis of RF signals using a delta-sigma modulator.
For high precision digital to analog conversion, when large oversampling is possible (e.g., audio or wireless baseband applications where the signal bandwidth is relatively low and oversampling of 10-1000 times is possible), such delta-sigma modulators have been successfully used to achieve high accuracy digital to analog conversion while using a one bit quantizer, a modulator (noise shaper) followed by an analog filter. The feedback paths of such delta-sigma modulators, however, are difficult to implement at higher sampling frequencies (e.g., 1 GHz and above). Due to the existence of a non-linear quantizer, the algorithm is difficult to parallelize and hence implement at lower clock frequencies. In addition, the input signal is typically limited to a fraction of the reference voltage, to ensure stability of the modulator (e.g., the technique is not very power efficient). Furthermore, high oversampling ratios (100 or more) are needed for higher resolutions.
A need therefore exists for improved methods and apparatus for direct synthesis of RF signals at higher sampling frequencies. A further need exists for stable modulators for direct synthesis of RF signals where the oversampling ratio is reduced and offers improved noise performance relative to delta-sigma modulators and that can be implemented at higher frequencies (hundreds of megahertz to several gigahertz). For example, GSM signals in many countries are centered around 800 MHz and WCDMA signals are centered around 2.1 GHz (base station transmitter).